1. Field of the Invention
The present invention relates generally to a mobile telecommunication system, and in particular, to a device and method for effectively tracking the location of a mobile telephone in multipath fading characterstics and the non-line-of-sight (NLOS) environment.
2. Description of the Related Art
A mobile telecommunication network allows a registered mobile subscriber to make a call to anyone anywhere and at any time. FIG. 1 illustrates the configuration of a typical mobile telecommunication network. As shown in FIG. 1, the mobile telecommunication network includes a plurality of base stations (BSs) 21 to 24 for providing mobile telecommunication service to a mobile subscriber through a mobile telephone 10, a base station controller (BSC) 30 for controlling the BSs 21 to 24, and a mobile switching center (MSC) 50 for connecting the BSC 30 to another BTS or a PSTN (Public Switched Telephone Network).
In a cellular mobile telecommunication network, the whole service area is divided into a plurality of coverage areas having respective base stations (BS) therein. Each BS coverage area is called a xe2x80x9ccell.xe2x80x9d An MSC controls these BSs so that a subscriber can continue his call without interruption while moving between different cells.
The MSC 50 can reduce the time required for calling a subscriber by locating the cell of the subscriber. In case of an emergency like a fire, or a patient needing first aid treatment, the mobile subscriber should be accurately located. Tracking the location of a mobile subscriber within the boundary of a cell in a mobile telecommunication network is known as xe2x80x9clocation service.xe2x80x9d
A mobile telephone can be located by the mobile telephone itself or through a mobile telecommunication network.
To locate the mobile telephone by itself, the mobile telephone is provided with a GPS (Global Positioning System) receiver to calculate its location in latitude and longitude coordinates based on the location information received from a satellite through the GPS receiver. The requirement of having the extra GPS receiver, however, increases the price and the size of the mobile telephone. Another shortcoming with this method is that the load on the mobile telephone is increased because it has to calculate its location.
As an alternative to locating the mobile telephone by itself, the mobile telephone calculates its location by a trigonometry based on the signals received from at least three BSs. This method also increases the price and the size of the mobile telephone due to the requirement of a separately procured signal receiver. Further, the mobile telephone has a higher load because it has to calculate its location, and sometimes it cannot operate compatibly with other BSs employing a different scheme.
In the case that the mobile telephone network locates the mobile telephone, at least three BSs receive a signal from the mobile telephone, calculate the distances between the BSs and the mobile telephone using the arrival time of the signals at the BSs, then determine the location of the mobile telephone using the trigonometry. This location service is provided generally by a location data processor 40 included in a BSC or in by independently provided device. Upon a request for service about the location of a specific mobile subscriber, i.e., a mobile telephone of a user, the BSC 30 selects the BSs 21, 22, and 23 surrounding the mobile telephone for use in the location service, and these selected BSs 21, 22, and 23 are ready for communication with the mobile telephone.
The mobile telephone network can calculate the location of the mobile telephone using the time of arrival (TOA) or the time difference of arrival (TDOA).
The TOA method calculates the distance of a mobile telephone and a BS based on the TOA of a signal transmitted from the mobile telephone at the BS. That is, it is assumed that the mobile telephone is located at the intersection point of three circles having the radius of the distances between the BSs and the mobile telephone.
The TDOA method assumes that the TDOAs of a signal transmitted from the mobile telephone at the three BSs define a set of points on a hyperbola, and the mobile telephone is located at the intersection point of at least three hyperbolas. The implementation of this method requires accurate synchronization of each BS, as compared to the TOA method. However, the burden of synchronization is negligible as all the CDMA BSs are already synchronized to one another using their GPS receivers.
As described above, the network tracks the location of a mobile telephone using a specific signal transmitted from the mobile telephone. However, the signal of the mobile telephone is often propagated to a BS in a path longer than the distance between the mobile telephone and the BS due to the multipath fading characteristic and the NLOS effects in a real mobile telecommunication environment. In this case, at least three circles or hyperbolas do not meet at one point but overlap each other over an area. Therefore, the location data processor 40 should detect the most likely point where the mobile telephone is located in the overlap area.
FIG. 2 illustrates a typical TOA method for locating a mobile telephone. As shown in FIG. 2, three circles 101, 102, and 103, whose radiuses are the distance between the mobile telephone 10 and at least three BSs 21, 22, and 23, are overlapped across an area indicated as 1. The mobile telephone 10 is located in the overlap area 1.
One approach to locating the mobile telephone 10 in the overlap area 1 is to use a common chord, as shown in FIG. 3. When at least three circles 111, 112, and 113 are overlapped over an area without meeting at one point, the mobile telephone 10 is considered to exist at the intersection point of three common chords 114, 115, and 116. The three common chords 114, 115, and 116 are defined by the intersection points of the circles 111, 112, and 113. A common chord is defined as a line connecting two points where two circles intersect.
The above method using the common chord is not very accurate in locating the mobile telephone except in the case where the mobile telephone is an approximate equal distance apart from the selected BSs and in a similar propagation environment to each respective BS. In the case that a first mobile telephone 11 is nearer to the first BS 21, as shown in FIG. 4, the procedure will be described by a way of example.
In FIG. 4, two circles 131 and 132 are drawn based on the TOAs of a signal transmitted from the first mobile telephone 11 at the first and the second BSs 21 and 22. A first common chord 133 is defined by the intersection between the circles 131 and 132. But if the path between the first mobile telephone 11 and the second BS 22 is in an NLOS condition and the path between the first mobile telephone 11 and the first BS 21 is in a line-of-sight LOS) condition, the common chord 133 is positioned far left from the actual location of the mobile telephone 11.
The effect is the same in the opposite case. If the path between the first mobile telephone 11 and the second BS 22 is in the LOS condition and the path between the first mobile telephone 11 and the first BS 21 is in the NLOS condition, the common chord 133 is also far right from the actual location of the mobile telephone 11.
As noted from the example shown in FIG. 4, the locating method using a common chord involves a huge location error unless the paths between the mobile telephone and each BS have the same propagation environment.
Another method of locating the mobile telephone in the overlap area 1 is to use a least square (LS) scheme. For details of the LS method, see J. J. Caffery and G. L. Stuber, xe2x80x9cSubscriber Location in CDMA Cellular Networksxe2x80x9d, IEEE Trans. on Vehi. Technol. VT-47, no. 2, pp. 406-415, May 1998, and H. Hashemi, xe2x80x9cPulse Ranging Radiolocation Technique and Its Application to Channel Assignment in Digital Cellular Radioxe2x80x9d, in Proc. of IEEE VTC ""91, 1991, pp. 675-680.
In the LS method, assuming that a mobile telephone at position (x0, y0) transmits a specific signal at time xcfx840, N BSs at positions (x1, y1), (x2, y2), . . . , (xN, yN) receive the signal at time xcfx841, xcfx842, . . . , xcfx84N, and a light speed is c, the location of the mobile telephone with respect to an ith BS is (x, y) satisfying fi(x, y, xcfx84)=0 in the following equation:
fi(x, y, xcfx84)=c(xcfx84ixe2x88x92xcfx84)xe2x88x92{square root over ((xixe2x88x92x)2+(yixe2x88x92y)2)}xe2x80x83xe2x80x83(1)
N circles are drawn by repeating the above procedure for the N BSs. To locate the mobile telephone, therefore, a function F(x, y, xcfx84) is defined as                               F          ⁡                      (                          x              ,              y              ,              τ                        )                          =                              ∑                          i              -              1                        N                    ⁢                                    α              i              2                        ⁢                                          f                i                2                            ⁡                              (                                  x                  ,                  y                  ,                  τ                                )                                                                        (        2        )            
where xcex1i2 is a weighting factor for an ith signal path and 0xe2x89xa6xcex1i2xe2x89xa61. A location data processor assigns a small xcex1i2 to an NLOS path between the BS and the mobile telephone and a large xcex1i2 to an LOS path between the BS and the mobile telephone, thereby reducing the location error caused by different propagation environments.
Therefore, the location data processor can track the location of the mobile telephone by obtaining the position (x, y) which minimizes Eq. 2. However, as Eq. 2 is non-linear, it takes a complicated process to directly obtain the intended value. Hence, an iteration method has been suggested in which an intended value is calculated by an approximation method. According to this method, Eq. 1 is expanded to a Taylor series and only the first-order terms of the Taylor series expansion are linearized (see, G. L. Turin, et. al., xe2x80x9cSimulation of Urban Vehicle Monitoring Systemsxe2x80x9d, IEEE Trans. on Vehi. Technol., VT-21, pp. 1-9, February 1972). Yet, the iteration method also has a distinctive shortcoming in that the converging operation involves errors if the mobile telephone is near the BS or the circle of the BS.
While a gradient descent method can be considered (see, J. J. Caffery and G. L. Stuber, xe2x80x9cSubscriber Location in CDMA Cellular Networkxe2x80x9d, IEEE Trans. on Vehi. Technol., VT-47, no. 3, pp. 406-415, May 1998), the problem of a longer converging time than the iteration method using a Taylor series should be solved before it can be put to wide use.
It is, therefore, an object of the present invention to provide a method of tracking the location of a mobile telephone using curves connecting the points where circles intersect one another, the circles"" radiuses being the distances between BSs and the mobile telephone.
The object of the present invention can be achieved by providing a device and method for tracking the location of a mobile telephone. According to the device for tracking the location of a mobile telephone, each of a plurality of base stations receives a predetermined signal from the mobile telephone and calculates the distance between the mobile telephone and the base station based on the time of arrival of the signal at the base station. A location data processor receives information about the distances from the base stations, draws circles with the radii being the distances and the coordinates of the base stations at the centers thereof around the base stations, and determines the location of the mobile telephone using location tracking curves connecting the intersection points of the circles.
The location data processor selects a pair of circles among the circles, draws a location tracking curve connecting the intersection points between the selected pair of circles, and draws a location tracking curve for each of the other pairs of circles. The location tracking curve derived from the selected circle pair is a part of a circle with a center within the circle corresponding to the base station with the smaller variances of the times of arrival of the received signal between the two base stations corresponding to the selected circle pair. The circles formed by the location tracking curves have the centers thereof on a line connecting the centers of the selected circle pair.
The location data processor locates the mobile telephone by averaging the coordinates of the intersection points among the location tracking curves of the selected circle pair.
In the method of tracking a mobile telephone in a mobile telecommunications network: (a) each base station nearer to a mobile telephone receives a predetermined signal from the mobile telephone and calculates the distance between the mobile telephone and the base station and the variances of time arrival of the signal at the base station; (b) a circle is drawn to have a radius being the distance and the coordinates of the base station at the center thereof around the base station; (c) a pair of the first and the second base stations is selected among the base stations and a plurality of location tracking curves connecting two intersection points between the circles corresponding to the first and the second base stations are drawn; (d) one of the location tracking curves is selected using the variances of the first and the second base stations; (e) the steps (c) and (d) are repeated for the other pairs of the base stations; (f) the intersection points are obtained among the location tracking curves selected in step (d) and (e); and, (g) the location of the mobile telephone is determined using the coordinates of the intersection points obtained in step (f).
The plurality of location tracking curves are parts of circles with centers near to the base station with smaller variances between the first and the second base stations. The circles formed by the location tracking curves have the centers thereof on a line connecting the coordinates of the first and the second base stations.
The larger variances between the variances of the first and the second base stations are compared to the variances of the plurality of location tracking curves, and one of the location tracking curves is selected according to the comparison result.
The location coordinates of the mobile telephone are determined by averaging the coordinates of the intersection points obtained in step (f).
The present invention locates the location of a mobile telephone by drawing a plurality of circles with the radii being the distances between a mobile telephone and a plurality of base stations and the base stations at their centers, and using location tracking curves connecting the intersection points between each circle pair instead of the common chords defined by the circles.